According to the  section 37 of the Aliens Act (Ulkomaalaislaki 301/2004)

 Finnish citizen’s foreign family members are :

1. the spouse of a Finnish citizen.
2. a person in a registered partnership with a Finnish citizen.
3. an unmarried child under 18 years of age, whose guardian or the spouse of the guardian resides in Finland.
4. if the person residing in Finland is a minor, his or her guardian is the family member

Persons living in a marriage-like relationship and sharing the same household  provided that they have lived together for at least two years.

The two-year requirement does not apply if the persons have a child in their joint custody or if there are other weighty reasons.

Residence permit application by a family member  :

If you are a family member of a Finnish citizen and you intend to travel to Finland for a period of time exceeding three months

please follow the instructions:

• Apply for a residence permit at a Finnish mission either in your home country or in the country of departure where you have a legal residence well in advance.
• The residence permit application must be submitted